Japanese Unexamined Patent Application Publication No. 2008-120227 describes a technique of manufacturing a bumper reinforcement as a vehicular shock absorber that includes manufacturing a formed body having a constant cross-sectional shape by roll forming a high-tensile steel plate, then high-frequency hardening the formed body, and bending (sweep forming) into an arc shape such that it has a certain curvature in the longitudinal direction (the direction perpendicular to the cross-section) thereof.
Japanese Unexamined Patent Application Publication No. 2007-30023 describes a roll forming apparatus including a pair of rotary rolls disposed to sandwich a conveying path of a metal thin plate, and a rotary roll disposed thereunder, in which the distance between the pair of rotary rolls is variable, and the rotary roll disposed thereunder is movable up and down, and as a method of manufacturing a part using the apparatus, a technique of manufacturing a roll-formed part not having the same cross-section with respect to the longitudinal direction, in which when a part of a long metal thin plate is being conveyed between the pair of rotary rolls, the distance between the pair of rotary rolls is changed, and the rotary roll installed thereunder is moved up and down such that the metal thin plate being conveyed is locally deformed.
In the technique of Japanese Unexamined Patent Application Publication No. 2008-120227, a curvilineal closed cross-section part is manufactured by forming a closed cross-section part having a constant cross-sectional shape by a conventional roll forming, then high-frequency hardening, and giving a curvature with three benders. However, a part whose cross-sectional shape in the longitudinal direction of the part is not constant cannot be formed. Providing a curvature is limited to two-dimensional directions.
The technique of Japanese Unexamined Patent Application Publication No. 2007-30023 is a forming technique capable of varying the cross-sectional shape in the longitudinal direction of the part using a forming apparatus including, in addition to a conventional roll forming having a pair of rotary rolls, a rotary roll to vary the cross-sectional shape in the longitudinal direction of the part located under the pair of rotary rolls. However, it is impossible to simultaneously provide a curvature in the longitudinal direction.
As described above, in the conventional techniques, it is impossible to simultaneously perform varying of cross-sectional shape in which a cross-sectional shape (specifically, the cross-sectional shape in a direction perpendicular to the longitudinal direction of the part, that is, in the width direction of the part) is varied in the longitudinal direction of a part, and giving of a three-dimensional shape in which a curvature in the longitudinal direction of the part (or, in addition thereto, a twist). Therefore, varying of a cross-sectional shape and giving of a three-dimensional shape to a part need to be performed as separate forming processes, and the cost is thereby increased. Further, when a high strength steel plate is used as a material for the purpose of weight reduction, a forming defect is likely to generate during the bending after the varying of cross-sectional shape of the part.